Multichannel interferometer with phase generated carrier demodulation and quadrature error correction

ABSTRACT

Apparatus for eliminating sign uncertainty in a coherent phase generated carrier demodulator in a multi-channel sensor system has a downconverter array arranged to separate the in-phase component I and the quadrature phase component of the sensor output for each channel. A coordinate transformer uses the in-phase component and quadrature phase component to calculate an arctangent for the phase angle for each channel. A digital signal processor adds 180° to each arctangent calculation for which the tangent is a negative number.

CROSS REFERENCE TO RELATED APPLICATION

[0001] Applicants claim the benefit of U.S. Provisional ApplicationSerial No. 60/220,266, filed Jul. 24, 2000 for System for DeterminingPhase Offset in a Numerically Controlled Oscillator to Eliminate SignUncertainty in a Phase Generated Carrier Demodulator.

BACKGROUND OF THE INVENTION

[0002] This invention relates generally to signal processing techniquesfor fiber optic sensor systems. This invention relates particularly todemodulation of signals output from an array of fiber opticinterferometric sensors for determining changes in a physical parametermeasured by the individual sensors. Still more particularly, thisinvention is directed to determining the phase offsets in a numericallycontrolled oscillator for the fundamental carrier and carrier firstharmonic in order to eliminate the sign uncertainty in a coherent phasegenerated carrier demodulator.

[0003] Mismatched fiber optic interferometers are commonly used assensing elements in fiber optic sensor arrays for measuring changes in aparameter such as fluid pressure, acceleration, magnetic fieldintensity, etc. Such sensing elements measure the time-varying phasedelay between optical signals that have propagated along separateoptical paths having unequal path length.

[0004] Mixing between a reference signal and a data signal is oftennecessary to extract information from an optical carrier. Ininterferometric sensing the mixing is typically between a referencesignal and a signal whose phase has been modified, or modulated by theparameter being measured.

[0005] Modulation is commonly used to transmit information from aninformation source, such as a sensor system where information isdetected, to an information destination, such as a receiver system wheredetected signals are received and processed. According to conventionalmodulation techniques, a signal of interest detected by a sensormodulates a carrier signal by modifying one or more characteristics ofthe carrier signal, such as amplitude, frequency or phase, to form amodulated carrier signal. The modulated carrier signal is then moreeasily transmitted over the appropriate communication channels to thedestination or receiver system where the modulated carrier signal isdemodulated to recover the signal of interest.

[0006] The fiber optic sensors detect or sense signals that modulate theoutput phase of the sensor system or interferometer. The modulatedcarrier can then be transmitted to a receiver system and photodetected.In a system having an array of sensors, the signals are oftenmultiplexed, for example, using time division multiplexing (TDM) and/orwavelength division multiplexing (WDM), as well as frequency divisionmultiplexing (FDM).

[0007] Fiber optic sensor systems acquire in the demodulation processone term proportional to the sine of the sensor phase shift and anotherterm proportional to the cosine of the sensor phase shift. The sine ofthe sensor phase shift is referred to as the quadrature term, Q; and thecosine of the sensor phase shift is referred to as the in-phase term, I.The angle of the phase shift is determined by calculating the ratio I/Q,which is the arctangent of the sensor phase shift. The amplitudes of thesine and cosine terms must be set equal by a normalization procedure toensure accurate implementation of an arctangent routine to determine thesensor phase shift.

[0008] One type of modulation technique used in interferometers andother sensing systems involves the use of phase generated carriers. Thetime varying phase signal (signal of interest) of each sensor modulatesthe phase generated carriers to form modulated carriers. Both the signalof interest and the phase generated carriers can be mathematicallyrepresented as a Bessel series of harmonically related terms. Duringmodulation, the Bessel series of the signals of interest modulates theBessel series of the phase generated carriers. The number of terms inthe Bessel series of the resulting modulated carriers will be dependentupon the amplitude of the measured or detected signals of interest. Theharmonically related terms in the Bessel series of the modulatedcarriers represent both the measured or detected signals of interest andthe carrier signals.

[0009] Typical fiber optic sensor systems using phase generated carriersto transmit a detected or measured signal (signal of interest) to areceiver system have used a pair of quadrature carriers with frequenciesof either ω_(c) and 2ω_(c) or 2ω_(c) and 3ω_(c), where ω_(c) is thephase generated carrier frequency. In multiplexed sensor systems, thesensor sampling frequency f_(s) must be selected to ensure thatfrequencies greater than f_(s)/2 are not aliased into the band ofinterest below f_(s)/2.

[0010] In some systems the optical signal input to the interferometer isa phase generated carrier produced by producing time-dependentvariations in the frequency of the optical signal output by a laser. Aphase generated carrier may be produced by various techniques. One suchtechnique involves routing the source output through a phase modulatorand applying a sequence of separate and different linear ramp voltagesto the linear phase modulator to produce step changes in the opticalfrequency.

[0011] In some systems the optical signal input to the interferometer isa phase generated carrier produced by generating time-dependentvariations in the frequency of the optical signal output by a laser. Aphase generated carrier may be produced by various techniques. Onetechnique involves routing the laser source output through an externalphase modulator and applying a sequence of separate and unique linearramp voltages to the linear phase modulator to produce step changes inthe optical frequency.

[0012] Another technique for producing a phase generated carrier usessinusoidal phase modulation of the source signal. Instead of samplingsignals associated with separate optical frequencies, the sampling ofsignals is associated with integration over portions of a period of thephase generated carrier.

[0013] Still another technique for producing a phase generated carrierinvolves the use of a Direct Digital Synthesizer (DDS) containing anumerically controlled oscillator (NCO). In particular, carriers thatare 180° out of phase with the NCO phase will produce sensor responseswith opposite sign after demodulation different than those produced bycarriers that are in phase with the NCO phase in the DDS. Whencoherently combined, sensor responses with opposite signs will combinedestructively, which results in an attenuation of the combined outputand a reduction in overall system dynamic range.

SUMMARY OF THE INVENTION

[0014] The present invention significantly increases the dynamic rangeof the coherent phase generated carrier demodulator by reducing signalattenuation that is caused when individual sensor responses of oppositesign in a synchronous environment are coherently combined.

[0015] Apparatus according to the invention for reducing signuncertainty in a coherent phase generated carrier demodulator in aninterferometric acoustic sensor system, comprises an optical signalsource that provides a phase generated carrier signal to the acousticsensor system so that the multi-channel acoustic sensor system producesan acoustic signal that is superimposed on the phase generated carriersignal, the interferometric acoustic sensor system being arranged toprovide an optical signal output that includes the phase generatedcarrier signal and the acoustic signal. The invention further comprisesa photodetector arranged to receive the optical signal output from theinterferometric acoustic sensor system and a downconverter connected tothe photodetector. The downconverter is arranged to separate an in-phasecomponent I and a quadrature component Q of the acoustic signal from thephase generated carrier signal. The invention also includes a coordinatetransformer connected to the downconverter. The coordinate transformeris arranged to function as a rectangular to polar converter and providesignals indicative of a polar phase angle between the in-phase componentI and the quadrature phase component Q of the acoustic signal.

[0016] A method according to the invention for eliminating signuncertainty in a coherent phase generated carrier demodulator in amulti-channel sensor system comprises the steps of arranging an opticalsignal source to provide a phase generated carrier signal to themulti-channel acoustic sensor system so that the multi-channel acousticsensor system produces in each channel an acoustic signal that issuperimposed on the phase generated carrier signal and connecting anarray of downconverters to the photodetector array. The method furthercomprises the steps of arranging the array of downconverters to separatean in-phase component I and a quadrature component Q of the acousticsignal from the phase generated carrier signal in each channel andconnecting a coordinate transformer to the array of downconverters. Themethod also includes the step of arranging the coordinate transformer tofunction as a rectangular to polar converter and provide signals thatindicate a polar phase angle between the in-phase component I and thequadrature phase component for each channel.

[0017] The invention preferably further comprises the steps of adjustinga phase register in each downconverter in a predetermined number ofphase intervals starting at 0° and ending at 180° and sampling signalsoutput from each channel of the sensor system a predetermined number oftimes for each phase interval. The method also preferably includes thesteps of saving maximum values of the in-phase component I and thequadrature phase component Q, saving phase values that correspond to themaximum values of the in-phase component I and the quadrature phasecomponent Q, and setting each downconverter to the phase value thatproduced the stored maximum values of the in-phase component I and thequadrature phase component Q for the corresponding channel.

[0018] The invention preferably also further comprises the steps ofcalculating the difference of successive squares of Q to determine aquantity D_(Q)=(Q_(i))²−(Q_(i)−1)², calculating the difference ofsuccessive squares of Q to determine a quantityD_(I)=(I_(i))²−(I_(i)−1)², summing the quantities D_(Q) and D_(I) todetermine a sum term D_(Qs) and a sum term D_(Is), calculating aquantity ${R = \left( \frac{D_{Qs}}{D_{IS}} \right)^{0.5}},$

[0019] adjusting the phase generated carrier gain by an amountproportional to 1−R; and repeating the preceding steps until R≦0.1.

[0020] The invention preferably further comprises the steps of samplingsignals output from each channel of the sensor array a predeterminednumber of times, saving the maximum values of Q and I, calculating theratio R_(t) of the maximum value of Q to the maximum value of I, andadjusting the signals Q and I if the ratio of their maximum valuesdiffers from unity.

[0021] An appreciation of the objectives of the present invention and amore complete understanding of its structure and method of operation maybe had by studying the following description of the preferred embodimentand by referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 illustrates a fiber optic sensor system;

[0023]FIG. 2 is a block diagram of a direct digital synthesizer that mayincluded in the fiber optic sensor system of FIG. 1;

[0024]FIG. 3 is a block diagram of the apparatus of the presentinvention for processing signals output from the fiber optic sensorarray of FIG. 1;

[0025]FIG. 4A is a flow chart of an algorithm used with the apparatus ofFIG. 3 to eliminate uncertainty in the sign of the sensor response; and

[0026]FIG. 4B is a flow chart illustrating additional details of thealgorithm of FIG. 4A.

[0027]FIG. 4C is a flow chart illustrating additional details of thealgorithm of FIG. 4B.

DETAILED DESCRIPTION OF THE INVENTION

[0028] This invention is directed to a signal processing algorithm forprocessing signals output from a sensor. FIG. 1 illustrates amulti-channel fiber optic sensor system 10 with which the algorithmaccording to the present invention may be used. The algorithm accordingto the present invention may be used with other sensor architectures(not shown) and with other polarization diversity detectors (not shown).The particular fiber optic sensor system 10 is disclosed herein only toprovide an example of such apparatus that may be used with theinvention.

[0029] The fiber optic sensor system 10 is fully disclosed in U.S.patent application Ser. No. 09/429,048, filed Oct. 29, 1999 and assignedto Litton Systems, Inc., assignee of the present invention. The fiberoptic sensor system 10 is also fully disclosed in U.S. patentapplication Ser. No. 09/430,057, filed Oct. 29, 1999 and assigned toLitton Systems, Inc.

[0030] The fiber optic sensor system 10 includes a plurality of lasers12-17 arranged to supply a plurality of corresponding optical feed lines20-25. The optical feed lines are joined at an optical terminator 28that is connected to a fiber optic sensor array 29. The opticalterminator 28 is connected to a downlead cable 30, which is connected toan acoustic array cable 32. The acoustic array cable 32 houses aplurality of sensors, which in this exemplary embodiment totalninety-six and are designated S1-S96. The optical terminator 28 alsoprovides a link between the downlead cable 30 and a plurality (e.g., 16)of return fibers 34-49, which are arranged to provide optical signals tocorresponding tricell photodetectors 50-65. The outputs of the tricellphotodetectors 50-65 are electrically connected to a system processor68.

[0031] A plurality of phase modulators 76-81 are arranged to modulatethe optical signals output from the lasers 12-17, respectively. Each ofthe lasers 12-17 generates an optical signal having a different opticalwavelength. Preferably, the phase modulators 76-81 are eachcharacterized by a different modulation frequency. Accordingly, thelasers 12-17 produce six optical signals, each having different opticalwavelengths and each modulated at a separate modulation frequency.

[0032] A direct digital synthesizer (DDS) array 84 provides oscillatorysignal inputs having frequencies f₁-f₆ to the phase modulators 76-81,respectively. Analog Devices AD9850 CMOS 125 MHz DDS is suitable for usein the present invention. The basic structure of a DDS 84 is shown inFIG. 2. As shown in FIG. 2 a data input register 92 receives serial andparallel load inputs, which are input to a frequency/phase data register94 under the control of a work load clock signal. The frequency/phasedata register 94 produces a 32-bit tuning word and phase and controlwords that are input to a high speed DDS 96 under the control of afrequency update data register reset signal. The high speed DDS 96receives reference clock in and master reset signals and provides anoutput to a digital to analog converter (DAC) 88 that is arranged toprovide an analog signal output. The DDS array also includes acomparator 90 that receives analog inputs and provides clock out signal.The DDS 84 is a highly integrated device that uses advanced DDStechnology coupled with the internal high speed, high performance,digital to analog (D/A) converter 88 and the comparator 90 to form acomplete digitally programmable frequency synthesizer and clockgenerator. The DDS 86 is used to generate a spectrally pure,frequency/phase programmable analog sine wave.

[0033] As shown in FIG. 1, there are six phase modulators 76-81connected to the DDS array 84. Accordingly, the array 84 includes theDDS 84 and five additional DDS's (not shown) that are identical instructure to the DDS 84. The DDS array 84 drives the phase modulators76-81 to provide six phase generated carriers that are input to thesensor array 29.

[0034] The sensors S1-S96 may be formed as Michelson interferometers(not shown) or Mach-Zehnder interferometers (not shown) that produceinterference patterns in response to a changes in a parameter beingmonitored by the sensor system 10. For example the parameter may beacoustic pressure. The prior art is replete with examples of such fiberoptic interferometric sensors used to monitor physical parameters.

[0035] Referencing FIG. 3A, each channel of the output of the sensorsystem includes a digital down converter 110 that separates the acousticsignal from the phase generated carrier. The digital down converter 110includes a plurality of mixers 112-115 shown in FIG. 3B. The mixers112-115 receive signals from the sensor system 10.

[0036] The sensor system 10 uses phase generated carriers havingfrequencies f₁ and f₂. Each of the mixers 112-115 is connected to thesystem processor 68, which is a digital signal processor (DSP). The DSP68 provides a first signal to each of the mixers 112-115 to indicate theNCO frequency. The NCO frequency signals preferably are 32 bit digitalsignals. The DSP 68 provides a second signal to each of the mixers112-115 to indicate the NCO phase. The NCO phase signals preferably are16 bit digital signals.

[0037] The input to the mixer 112 is a superposition of the sensorsystem 10 output and the carrier frequency f₁. The input to the mixer113 is a superposition of the sensor system 10 output and the firstharmonic of the carrier frequency f₁. The input to the mixer 114 is asuperposition of the sensor system 10 output and the carrier frequencyf₂. The input to the mixer 113 is a superposition of the sensor system10 output and the first harmonic of the carrier frequency f₂.

[0038] The individual sensor responses are superimposed on the carrierand the carrier first harmonic, which are downconverted and filtered.The in-phase and quadrature components of the individual sensorresponses are converted to polar phase by a coordinate transformer 122by implementing an arctangent function.

[0039] The mixers 112-115 provide signal outputs to corresponding lowpass filters 116-119. The output of the low pass filter 116 is a signalI₁ that indicates the in-phase signal component for the first carrierfrequency f₁. The output of the low pass filter 117 is a signal Q₁ thatindicates the quadrature signal component for the first carrierfrequency f₁. The output of the low pass filter 118 is a signal I₂ thatindicates the in-phase signal component for the second carrier frequencyf₂. The output of the low pass filter 119 is a signal Q₂ that indicatesthe quadrature signal component for the second carrier frequency f₂.

[0040] The signals I₁, Q₁ output from the lowpass filters 116 and 117,respectively, are input to a coordinate transformer 122 that functionsas a rectangular to polar converter. The signals I₂, Q₂ output from thelowpass filters 118 and 119, respectively, are also input to thecoordinate transformer 122, which calculates the polar phase angle foreach channel using the arctangent function. The coordinate transformer122 provides signals that indicate the arctangents to the DSP 68, whichdetermines the quadrant for each arctangent.

[0041] It should be noted that although only the carriers f₁ and f_(s)are discussed in detail, the sensor system 10 as illustrated in FIG. 1also includes carriers f₃, f₄, f₅ and f₆ that are arranged and processedin the same manner as described for the carriers f₁ and f₂. It shouldalso be noted that the invention is not limited to any specific numberof carriers.

[0042] The sensor output signals Q and I are sampled. As explained ingreater detail below, the digital signal processor 68 sets the initialNCO phase offsets to maximize the amplitudes of Q and I coming out ofthe corresponding demodulator. The digital signal processor 68 then setsthe depth of modulation in the corresponding optical signal source toproduce a coarse quadrature relationship between the fundamental andfirst harmonic. The differences of the squares of successive values of Qand I are calculated and summed to produce differences values D_(qs) andD_(is). The phase modulator voltage is adjusted to keep the square rootof the ratio D_(qs)/D_(is) less than unity.

[0043] The processor then fine-tunes the quadrature relationship betweenthe fundamental and first harmonic by balancing the gains of thein-phase and quadrature components of the demodulator output on achannel-by-channel basis. This is done by comparing the ratio of theirpeak values to unity and adjusting the gains of the signals Q and I areto maintain the signals at the same magnitude.

[0044] Referring to FIG. 4B, having established a true quadraturerelationship between the fundamental and the first harmonic, theprocessor scans the arctangent output on a channel-by-channel basis. Inthe event the arctangent output of a particular channel is negative, theprocessor adds π radians (180°) of phase offset to the fundamental NCOfrequency and adjusts the phase offset of the first harmonic NCO tomaintain the quadrature relationship between the two.

[0045]FIG. 4A illustrates the details of an algorithm according to theinvention for a sensor array that has four banks of interferometricsensors that each includes six sensor channels. After starting, thealgorithm executes a select bank step 130 and repeats the select bankstep 130 for each of six channels 132. A set calibration channel step133 is then executed for each bank of sensors. The algorithm thenproceeds with a set NCO phase to zero step 134. A wait step 135 thenfollows while a selected number of samples (e.g., 1000) are collected.The algorithm next has a collect samples step 136 in which a number ofsamples (e.g., 3713) are collected. A first save step 137 is executed inwhich maximum values of the in-phase component I and the quadraturephase component of the signal output from each sensor are saved. Asecond save step 138 is then performed in which the phase angles for themaximum values of the in-phase component I and the quadrature phasecomponent are saved. The algorithm proceeds with an increment phaseregister step 139. A test phase step 140 then determines whether thestored phase is 180°. If the phase is 180°, the algorithm repeats thewait step 135 and following steps 136-140 until the phase is not 180°.If the phase is not 180°, the algorithm returns to step 132 and selectsa new channel.

[0046] After steps 132-140 are completed for all six channels, then foreach of the six channels the algorithm has a set I phase register step142 in which an in-phase register is set to the phase corresponding tothe maximum value I_(max) of the in-phase component. The algorithm alsoincludes a set Q phase step 144 in which a quadrature-phase register isset to the phase corresponding to the maximum value Q_(max) of thequadrature phase component. After the steps 142 and 144 have beencompleted for all six channels in the selected bank, the algorithmproceeds with the steps shown in FIG. 4B.

[0047] Referencing FIG. 4B, the algorithm has a set calibration step 150in which a calibration channel is set to the current signal channel. Await step 151 is then performed for a number of samples (e.g., 1000) ofthe sensor output. After the wait step 151, the algorithm does a selectsample step 152 in which samples are selected at a regular interval forfurther processing. For example, the select step 152 may select every128^(th) sample until 3713 samples have been collected. As each sampleis collected, a save step 153 is performed to save the maximum values ofthe in-phase component I_(max) and the quadrature phase componentQ_(max) of the sensor output. The algorithm then proceeds with adifferencing step 154 in which the difference between the squares ofsuccessive selected samples of the quadrature phase componentD_(Q)=(Q_(i))²−(Q_(i)−1)² and the in-phase componentD_(I)=(I_(i))²−(I_(i)−1)² are calculated. In the difference calculationsthe subscript “i” denotes the number of the selected sample for whichcalculations are being done.

[0048] The algorithm includes a summing step 144 in which sumsD_(Qs)=sum(D_(Q)) and D_(Is)=sum(D_(I)) are calculated. These sums areinput processed by a reference calculation step 156. The step 156calculates a gain reference $R_{t} = \frac{I_{peak}}{Q_{peak}}$

[0049] and also calculates a modulation depth reference$R = {\left( \frac{D_{Qs}}{D_{IS}} \right)^{0.5}.}$

[0050] After steps 153-156 are executed for the selected number ofsamples, a gain adjust step 158 tests the gain reference R_(t) todetermine whether it is less than or greater than unity. If R_(t)<1, thegain is adjusted to increase the quadrature phase component Q. If thegain reference R_(t)>1, the gain is adjusted to increase the in-phasecomponent I. A modulation depth step 159 is also done by comparing themodulation depth reference to unity. If modulator depth R<1, then themodulator depth is increased. If the modulator depth R>1, then themodulator depth is decreased.

[0051] After these calculations have been completed for the entiresensor system 10, which may include four banks that each have sixchannels, the algorithm proceeds to execute the steps of FIG. 4C.Referencing FIG. 4C, a collect channel phase data step 160 receives thechannel arctangent results 162. A test step 164 determines whether thearctangent is negative. If the arctangent is negative, an increment step166 adds 180° to an offset register corresponding to the negativearctangent. If the arctangent is not negative, then a test step 168determines whether all the arctangents for the sensor array 10 have beenconsidered. For the particular sensor system 10 disclosed herein thesteps 160, 162, 164 and 166 are performed twenty-four times.

[0052] The sensor output signals Q and I are sampled. The digital signalprocessor 68 sets the initial NCO phase offsets to maximize theamplitudes of Q and I coming out of the corresponding demodulator. Thedigital signal processor 68 then sets the depth of modulation in thecorrespond optical signal source to produce a coarse quadraturerelationship between the fundamental and first harmonic. The differencesof the squares of successive values of Q and I are calculated and summedto produce differences values D_(qs) and D_(is). The phase modulatorvoltage is adjusted to keep the square root of the ratio D_(qs)/D_(is)less than unity.

[0053] The processor then fine-tunes the quadrature relationship betweenthe fundamental and first harmonic by balancing the gains of thein-phase and quadrature components of the demodulator output on achannel-by-channel basis. This is done by comparing the ratio of theirpeak values to unity and adjusting the gains of the signals Q and I areto maintain the signals at the same magnitude.

[0054] Referring to FIG. 4B, having established a true quadraturerelationship between the fundamental and the first harmonic, theprocessor scans the arctangent output on a channel-by-channel basis. Inthe event the arctangent output of a particular channel is negative, theprocessor adds π radians of phase offset to the fundamental NCOfrequency and adjusts the phase offset of the first harmonic NCO tomaintain the quadrature relationship between the two.

What is claimed is:
 1. Apparatus for reducing sign uncertainty in a coherent phase generated carrier demodulator in an interferometric acoustic sensor system, comprising: an optical signal source that provides a phase generated carrier signal to the interferometric acoustic sensor system so that the multi-channel acoustic sensor system produces an acoustic signal that is superimposed on the phase generated carrier signal, the interferometric acoustic sensor system being arranged to provide an optical signal output that includes the phase generated carrier signal and the acoustic signal; a photodetector arranged to receive the optical signal output from the interferometric acoustic sensor system; a downconverter connected to the photodetector, the downconverter being arranged to separate an in-phase component I and a quadrature component Q of the acoustic signal from the phase generated carrier signal; and a coordinate transformer connected to the downconverter, the coordinate transformer being arranged to function as a rectangular to polar converter and provide signals indicative of a polar phase angle between the in-phase component I and the quadrature phase component Q of the acoustic signal.
 2. The apparatus of claim 1 wherein the coordinate transformer provides signals that indicate an arctangent of the polar phase angle between the in-phase component I and the quadrature phase component Q.
 3. The apparatus of claim 1 wherein the interferometric acoustic sensor system comprises a multichannel interferometric sensor array.
 4. The apparatus of claim 3, further comprising: a photodetector array having a plurality of photodetectors arranged in corresponding relationship to the channels of the multichannel interferometric sensor array; and an array of downconverters connected to the photodetector array, the array of downconverters being arranged to separate the in-phase component I and the quadrature phase component Q of the acoustic signal from the phase generated carrier signal for each channel in the multichannel interferometric sensor array.
 5. The apparatus of claim 1 wherein each downconverter in the array of downconverters comprises: a first mixer arranged to receive a superposition of signals output from the sensor system and a the phase generated carrier signal that is input to the sensor array by the optical signal source to form the in-phase component I; and a second mixer arranged to receive a first harmonic of a superposition of signals output from the interferometric acoustic sensor system and the phase generated carrier signal to form the quadrature phase component Q.
 6. The apparatus of claim 5, further comprising a plurality of optical signal sources connected to the multi-channel interferometric sensor array so that each channel therein receives a corresponding phase generated carrier signal.
 7. The apparatus of claim 6 wherein each of the phase generated carrier signals has a unique carrier frequency.
 8. The apparatus of claim 1, further comprising a digital signal processor is arranged to add 180° to each polar phase angle that is a negative number.
 9. The apparatus of claim 8 wherein the digital signal processor is arranged to normalize the in-phase and quadrature signals.
 10. A method for eliminating sign uncertainty in a coherent phase generated carrier demodulator in an interferometric acoustic sensor system, comprising the steps of: arranging an optical signal source to provide a phase generated carrier signal to the acoustic sensor system so that the interferometric acoustic sensor system produces an acoustic signal that is superimposed on the phase generated carrier signal; arranging the interferometric acoustic sensor system to provide an optical signal output that includes the phase generated carrier signal and the acoustic signal; arranging a photodetector to receive the optical signal output from the interferometric acoustic sensor system; connecting a downconverter to the photodetector; arranging the downconverter to separate an in-phase component I and a quadrature component Q of the acoustic signal from the phase generated carrier signal; and connecting a coordinate transformer to the downconverter, the coordinate transformer being arranged to function as a rectangular to polar converter and provide signals indicative of a polar phase angle between the in-phase component I and the quadrature phase component Q of the acoustic signal.
 11. The method of claim 10 including the step of arranging the coordinate transformer to provide signals that indicate an arctangent of a polar phase angle between the in-phase component I and the quadrature phase component for each channel.
 12. The method of claim 10, further comprising the steps of: adjusting a phase register in each downconverter in a predetermined number of phase intervals starting at 0° and ending at 180°; sampling signals output from interferometric acoustic sensor system a predetermined number of times for each phase interval; saving maximum values of the in-phase component I and the quadrature phase component Q; saving phase values that correspond to the maximum values of the in-phase component I and the quadrature phase component Q; and setting the downconverter to the phase value that produced the stored maximum values of the in-phase component I and the quadrature phase component Q for the corresponding channel.
 13. The method of claim 12, further comprising the steps of: calculating the difference of successive squares of Q to determine a quantity D_(Q)=(Q_(i))²−(Q_(i)−1)²; calculating the difference of successive squares of Q to determine a quantity D_(I)=(I_(i))²−(I_(i)−1)²; summing the quantities D_(Q) and D_(I) to determine a sum term D_(Qs) and a sum term D_(Is); calculating a quantity $R = {\left( \frac{D_{Qs}}{D_{IS}} \right)^{0.5}.}$

adjusting the phase generated carrier gain by an amount proportional to 1−R; and repeating the preceding steps until R≦0.1
 14. The method of claim 13, further comprising the steps of: sampling signals output from the multichannel interferometric sensor array a predetermined number of times; saving the maximum values of Q and I; calculating the ratio R_(t) of the maximum value of Q to the maximum value of I; and adjusting the signals Q and I if the ratio of their maximum values differs from unity.
 15. The method of claim 12, further comprising the steps of: arranging a first mixer to receive a superposition of signals output from the sensor system and a selected phase generated carrier signal having frequency that is input to the sensor array by the optical signal source to determine the in-phase component I; and arranging a second mixer to receive a harmonic of a superposition of signals output from the multichannel interferometric sensor array and the selected carrier signal;.
 16. The method of claim 15 further comprising the steps of connecting a digital signal processor to the coordinate transformer and arranging the digital signal processor to add 180° to each arctangent calculation that yields a negative number.
 17. The method of claim 16, further comprising the steps of: forming the interferometric acoustic sensor system as a multichannel interferometric sensor array; providing a plurality of optical signal sources connected to the multi-channel acoustic sensor array so that each channel therein receives a corresponding phase generated carrier signal.
 18. The method of claim 17, further comprising the step of connecting a plurality of optical signal sources to the multi-channel interferometric sensor array so that each channel therein receives a corresponding phase generated carrier signal.
 19. The method of claim 18 further comprising the step of forming each of the phase generated carrier signals to have a unique carrier frequency. 